There are two major shield methods around: earth pressure balanced (EPB) and slurry type shield machine. Selection of shield method depends on ground conditions, surface conditions, dimensions of the tunnel section, boring distance, tunnel alignment and construction period. Both are closed-face type shield machines, meaning the "head" part of machine is "closed" and separated from the rear part of machine. The "head" has a working chamber filled with soil or slurry between the cutting face and bulkhead to stabilize the cutting face under soil pressure . The EPB type shield machine turns the excavated soil into mud pressure and holds it under soil pressure to stabilize the cutting face. It has excavation system to cut the soil, mixing system to mix the excavated soil into mud pressure, soil discharge system to discharge the soil and control system to keep the soil pressure uniform. Therefore, EPB may not be applicable for the rocky soil that is difficult to turn the excavated soil into slurry. It can be used at ground predominated by clayer soil. The slurry type shield machine, on the other hand, uses the external pressurized slurry to stabilize the cutting face, similar to bored piles or diaphragm walls using bentonite to contain the trench wall. The slurry is circulated to transport the excavated soil by fluid conveyance. Besides having excavation system, the slurry type shield machine has slurry feed and discharge equipment to circulate and pressurize slurry and slurry processing equipment on the ground to adjust the slurry properties.



A typical rail tunnel section with its components


Schematic construction sequence for TBM


A shield machine at factory ready to transport out to the site


Main components of a shield machine


Schematic representation of EPBM


Schematic representation of EPBM


Types of cutting face of EPBM


EPBM circulation system


Typical site utilization plan for EPBM including working shaft, gantry crane, segment stock yard, tanks, etc.


Section view of the EPBM site utilization


Schematic representation of a slurry type shield machine


Cutting head sketch of slurry type shield


Types of cutting face of slurry type shield


Slurry type TBM: Slurry circulation system


Slurry type TBM: Slurry circulation system


Slurry type TBM: Slurry circulation system


Slurry treatment plant


Typical site utilization plan for slurry type TBM including working shaft, gantry crane, segment stock yard, tanks, etc.


Section view of the site utilization for the slurry type TBM


Shaft construction using diaphragm wall method (C905)


Shaft construction using diaphragm wall method (C905)


Shaft construction using diaphragm wall method (C902). Access stairs, ventilation and passenger hoist are provided.


Main body of the shield machine being lowered down into launching shaft. From there, the TBM system is to be assembled and starts boring.


Lowering down the main body of shield machine into launching shaft


A receiving shaft to accommodate dismentle (right) and assemble (left) TBM at the same time (C902)


Assemble TBM (C902)


The shield machine at work


The function part of TBM (backup) at the rear (C902)

Work Sequece:

Lower main body, cutting head, segment erector, screw conveyer, tail shield, thrust frame > Assemble TBM > Erection of reaction frame > Connection of hose and cables > Installation of 4 to 5 nos. of temp segments > Initial drive > Completion of initial drive > Modification of TBM including removal of reaction frame, cradle and temp rings, laying rail > Main drive > Break through soft eye (reach receiving shaft) > Dismantle TBM

- Gantry crane (for example, 35ton, 45ton) is normally used crossing the shaft.
- Lower down TBM components to the shaft needs heavy lifting. 600ton or 700ton crawler crane is needed.
- TBM production rate depends on soil condition. Normally at 5-10 m/d.
- Ground improvement is necessary at receiving shaft end for TBM to pass through (about 3-4m in length). Start end depends on ground conditions.
- Grouting to the gap between shield skin and segment following segment installation from ground surface.
- Dismantle TBM can be done in launch shaft after being transporting back from receiving shaft end to the launching shaft end, leaving behind shield skin at receiving shaft end.



The start end of tunnel lining (C902)


Lining segments are being lowered down from shaft to the locomotive


Close view of segment lining. A typical ring of lining: 1.4m width with 6 segemnts to be joined by bolts. Note that the joint of neighbouring rings to be staggered (C902)


Locomotive rail (C902)


The locomotive. It transports muck out and segments to and fro from shaft to the TBM (C902)


Completed tunnel with linings. From here, 1st stage concrete, rail track, ventilation, electrical and other cables, etc., starts installation.



Pipe Jacking, a smaller scale "TBM" method to lay sewer lines


TBM breaks through soft eye


Backfill Grouting (tailskin grout): Grouting to fill void between the lining and the ground (tail void) by grout injection
Cycle Time: Segment erection > excavation / backfill grouting > segment erection
EPB: Earth Pressure Balance (Shield Machine)
Goliath Crane: Gantry crane
Segment, Ring and Lining:
Segment: a splice of precast concrete to form part of ring structure. This ring is also called primary lining. The inside surface is sprayed with concrete, usually for waterproofing and finishing. Also called secondary lining. Segment > Lining (primary, secondary), or ring
- To allow muck taken out, lining material brought in, TBM lowering in and assembly, reaction force at the start, operation space
- Launching shaft, intermediate shaft, receiving shaft, turning shaft

Screw Conveyor: Carry excavated rocks and earth from cutting face to the rear of the completed tunnel
Muck: Soil slurry
Overburden: Distance from top point of tunnel to the surface of ground. Usually 1 - 1.5D
TBM: Tunnel Boring Machine

1. Japanese standard for shield tunneling, third edition, 1996.
2. Mitsubishi Heavy Industries catalogue, 2005.
3. DTL 1 C902 (main contractor: Shanghai Tunnel), C905 (main contractor: Shimizu).




The New Austrian Tunneling Method (NATM), an alternative and cheaper way for tunneling.


NATM support tubes installation (double layer)


NATM support tubes installation (single layer)


NATM support system sketch


Basler&Hofmann's catalogue